mlxsw: spectrum_router: Hold a reference on RIF's netdev

Previous patches tried to make RIF deletion more robust and avoid
use-after-free situations.

As another precaution, hold a reference on a RIF's netdev and release it
when the RIF is deleted.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>